A question that comes up often is, “why should I use mechanical seals instead of packing when packing is cheaper?” There are some advantages that are obvious and easy to understand, and a number of others where the benefits may not be so readily apparent.
Why Use Packing?
First, it’s useful to look at why people use packing and why they might prefer it to a mechanical seal solution.
Pump packing is the earliest form of pump sealing and is still a widely used sealing technique. Originally gland packing was made from old ropes and natural fibre products that were packed around the shaft physically stuffing the gap with material to reduce leakage. This is where the name ‘stuffing box’ comes from.
Modern packing makes use of more sophisticated braiding and materials designed to wear less, reduce friction, and withstand heat better
- Maintenance personnel are familiar with the process of maintaining and re-packing packed glands.
- When packing fails it simply starts to leak more and so can be readjusted and tightened to reduce the leak with the pump remaining in service. Whereas mechanical seal failure can necessitate the shutdown of the pump. This is why pump packing is the primary source of packing for fire pumps and other applications that can’t risk catastrophic failure.
- The initial cost of Gland (Pump) Packing is low when compared to the costs associated with mechanical seal.
Why use a Mechanical Seal?
Cost of Product
Correctly adjusted gland packing should be maintained with a leakage rate of approximately 1-drop per minute of sealed product per inch/25mm of outside diameter of shaft. On a 2.000”/50mm diameter shaft this equates to 2 drops/min or 100 gals/month or 1200 gals/year (450 Lt/month or 5400 Lt/year).
These numbers are based on an ideal scenario and, as experience tells us, the leakage rates from packed glands are considerably higher than this, often by as much as 5-10 times.
Disposal of Effluent and Environmental Issues
Depending on the type of product the waste disposal costs can be significantly higher that the initial cost of manufacture. Just looking at our own domestic water bills illustrates this. Even when only dealing with water sewage, charges are significantly higher than the initial cost of the product.
Companies are also now under extreme pressure and constant observation on environmental issues Governments are constantly under national and global pressure to ensure that manufacturers accept the responsibility for the effects their products have on the environment and companies themselves often have environmental targets to meet. Leakage is becoming less and less acceptable and conservation of resources is increasingly important.
Mechanical seals reduce power consumption when compared to gland packing.
On average the majority of gland packing consumes 6 times more power than a balanced mechanical seal. Running rotating equipment with packing in the stuffing box is like driving your car with the hand brake on. Would this affect your fuel consumption? Of course it would and so packed glands therefore greatly increase power costs. In some cases pumps can be made to run at a lower speed and efficiency, increasing costs still further.
The single major cause of bearing failure in rotating equipment is contamination of the lubrication. In a study conducted by Mobil Oil it was discovered that water contamination of 0.002% reduces the rated bearing life by almost 50%. This is the equivalent of 2 drops of water in a litre of oil.
Packing leaks by design and is in close proximity with the bearings. This subjects the bearings to a constant stream of leakage and contamination. Any reduction in contamination will greatly increase the Mean Time Between Failure (MTBF), and in turn reduce maintenance costs.
No leakage equates to longer bearing life and less downtime costs. Many mechanical seal failures are attributed to excessive shaft movement caused by failed bearings.
Shaft / Sleeve Wear
Correctly designed mechanical seals do not wear expensive shafts or sleeves.
Most rotating equipment is fitted with repairable wear sleeves in the region of the gland packing to adress the fact that packing is designed to wear the shaft in this area. If a replaceable sleeve were not fitted, the shaft would need to be replaced or rebuilt in the worn area. This would be both expensive in terms of spares holding or extended downtime while shafts are repaired.
If a separate wear sleeve is fitted there is the cost of holding a spare in stock but there is an underlying danger that the shaft diameter under the sleeve has been greatly reduced to allow for fitting, considerably reducing the shafts resistance to bending.
Broken shafts are not uncommon and are generally caused by fatigue due to the constant bending of the shaft under operation. How does this happen? Running the pump away from its Best Efficiency Point, against throttled or closed discharge valves, over its designed capacity; long shaft overhang and stop-starting create massive forces within the volute.
It is then all down to the principle of levers and that thin things bend easier than fat things.
Shaft sleeves can also be notoriously difficult to remove. This can be extremely time consuming and may even lead to damage of the shaft itself.
Gland packing often requires the supply of a clean flushing medium to the lantern ring. This was originally done to prevent the ingress of air into a system under vacuum and was also found to cool the packing as it generated friction against the shaft.
The costs here are the supply of the flush, the leakage of the flush to atmosphere and the dilution of the product itself. This dilution may be acceptable or costs may be incurred in the removal of the dilutant at a later stage in the process.
No product dilution with mechanical seals means a better quality product and no additional costs to remove the dilutant.
To ensure that packing operates as efficiently as it should it requires constant attention. Too loose and leakage rates are unacceptable, too tight and the packing can burn out on start up, increasing leakage, wear on the shaft or sleeve and increasing power consumption. Packing is not simply just greasy rope but it is often treated as such. Maintenance must be constant and conscientious throughout the life of the packing to ensure acceptable operation.
Mechanical seals require no routine maintenance.
Eliminating leakage reduces secondary maintenance costs by reducing corrosion, cleaning and painting. Secondary maintenance costs would be greatly reduced.
Packing leaks, as it is designed to and it leaks the “sealed” product to atmosphere. As we have already agreed this is becoming more and more unacceptable for environmental reasons, especially if the product is corrosive, toxic or explosive
All products classified by fugitive emission or hazardous should be double sealed. Airborne pollution is greatly reduced; double seals can achieve zero product emissions.